Boiler Isolation

Check the piping direction first. A plate exchanger reaches a close approach only when the two streams run counter-current. Piped co-current - both fluids entering at the same end - the same unit cannot bring the boiler return down toward the building return, however many plates it has.

It is an easy mistake on a B25T or B120T because all four connections sit on one face and nothing looks wrong. The symptom is a boiler return that tracks well above the building return at every load. Swapping the two connections on one side corrects it without touching the unit.

They do not have to be equal, but the ratio decides which end of the exchanger gets the close approach. With equal flows the temperture difference is even along the plate. With less flow on the boiler side than the building side, the boiler water gives up more temperature and returns colder, which helps condensing but makes it harder to reach the building supply setpoint.

The opposite imbalance is the one to avoid: an oversized boiler pump sends water through too fast to cool and the return comes back hot. Give us both pump flows as they will actually run, not just the heat load.

Only if the boiler is seeing the return temperature it needs. A like-for-like replacement reproduces the old approach exactly, including a poor one. If the plant has since had a condensing boiler fitted, the original selection was probably made for a boiler that did not care.

Send the nameplate and the return temperature the boiler wants. Often the answer is the same frame with a higher plate count: the port positions stay where the pipework already is and only the pack depth and weight grow. If the frame is at its plate ceiling the duty moves up a frame, and then the connections need checking.

Rate it for the building side, because that is where the static head lands. The exchanger is there so the boiler does not carry that pressure, which means the exchanger does. Add the pump head and the relief setting to the static figure and give us the total as a design pressure.

Most hydronic work sits in the S class, published as 31 bar at 155 C. That letter is a category, not the stamped rating of a particular build, so we confirm the rating of the unit quoted against your design pressure. The pressure classes page sets out the other letters.

A strainer on the building-side inlet, sized to the particle limit of the frame, and isolation valves with flushing connections so the unit can be cleaned in place. Old loops carry rust sludge and scale fragments, and the first weeks after a new exchanger goes in are when most of it moves.

Fouling here does not announce itself as a heating complaint. It shows as the boiler return temperature creeping up over a season while the building stays warm, because the approach is widening. Log that temperature at commissioning so there is a baseline to compare against.